In a so called thermal transfer printing apparatus, the printhead includes a plurality of thermal heating elements which are selectively energisable by a controller during printing to warm and soften pixels of ink from the tape and to transfer such pixels to the substrate. Such printheads typically include a very large number of thermal printing elements, for example approximately 300 thermal printing elements per inch of the array, in order to be able to print high resolution images. The printhead presses the tape against the substrate such that the pixels of ink contact the substrate before the web of the tape is peeled away, thus transferring the pixels of ink from the tape to the substrate.
Such printing apparatus includes drive apparatus for moving the tape relative to the printhead, to present fresh tape, from which pixels of ink are yet to be removed, to the printhead, such that successive printing operations can be carried out. By enabling such movement and selectively energising the printing elements in each of a plurality of positions along the substrate and tape, a desired image can be built up from printed dots.
It has long been known to provide tape drives which include two spool supports, one of which supports a supply spool on which unused tape is initially wound, and the other of which supports a take-up spool, onto which the tape is wound after it has been used. Tape extends between the spools in a tape path. Each of the spool supports, and hence each of the spools of tape, is drivable by a respective motor.
It is known to provide thermal transfer printing apparatus in two different configurations. In the first, so called “intermittent” configuration, the substrate to be printed and the tape are held stationary during a printing operation, whilst the printhead is moved across the area of the substrate to be printed. Once the printing operation is complete, the printhead is lifted away from the tape, and the tape is advanced to present a fresh region of tape to the printhead for the next printing operation.
In the second, so called “continuous” configuration, the substrate to be printed moves substantially continuously and the tape is accelerated to match the speed of the tape before the printhead is brought into thermal contact with the tape and the printing operation is carried out. In this configuration, the printhead is maintained generally stationary during each printing operation.
The tape used in thermal transfer printers is thin. Therefore it is important to ensure that the tension in the tape extending between the two spools is maintained at a suitable value or within a suitable range of tensions, in particular to enable the web to peel cleanly away from the heated ink. Too much tension in the tape is likely to lead to the tape being deformed or broken, whilst too little tension will inhibit the correct operation of the device. A slack tape is likely to affect print quality.
In the case of a printing apparatus in continuous configuration, it is also necessary to accurately control the speed of the tape, to ensure that it matches the speed of the substrate. A typical thermal transfer printer operates with substrate that advances at linear speeds between approximately 0.01 meter per second and approximately 2 meters per second. Typical substrate accelerations are up to approximately 12 meters per second per second.
In order to avoid wasting tape, whilst maintaining acceptable print quality, it is advantageous to be able accurately to control the movement of the tape, so as to position the next portion of tape to be used directly adjacent a portion of the tape from which the ink has previously been removed. It is desirable for a spacing between adjacent regions of tape from which pixels are removed to create an image, to be less than 1 mm.
It is also important to ensure that the regions of tape from which ink is removed during successive printing operations do not overlap, so that the printhead does not attempt to remove ink from a region of the tape from which the ink has already been removed. However, it is known to interlace images, such that a previously used region of tape is reused, but in the second and/or subsequent printing operations, different pixels of ink are removed from the tape to create an image. Such a method is described in the applicant's earlier patent, GB2289441, also published as U.S. Pat. No. 5,908,251.
Tape drives of various types have been proposed, for example a tape drive which includes a stepper motor for driving a take up spool so as to pull tape through along a tape path between a supply spool and the take up spool. Such a tape drive also includes a mechanical clutch for setting and maintaining the tension in the tape. Such tape drives are mechanically complex and regular maintenance of the clutch is required. Furthermore, since the supply spool is operated at a fixed torque, the tension in the tape varies as the diameter of the supply spool varies over time.
Another example of a known tape drive is one in which a take up spool and a supply spool are rotated by respective stepper motors. The stepper motors are driven in a co-ordinated manner to transfer the tape from the supply spool to the take up spool and to accurately position the tape adjacent the printhead, whilst maintaining the tension in the tape. Various methods of determining and maintaining the tension of the tape have been proposed. Such methods require the measured tension in the tape to be compared with the desired tension, and for a correction to be applied. Therefore, such methods incur a delay of at least one printing operation between the tension in the tape falling outside an acceptable range and the correction being applied.
Stepper motors have a limited update rate of the motor, owing to the inherent step size of the motor. For example, a typical stepper motor has a maximum resolution of 3200 microsteps per revolution of the motor. This limits the ability of the motor control system to accurately position the tape, which in turn sets a minimum spacing between adjacent regions of tape from which ink can be removed, which the motor control system is able to achieve. It is only possible to make a change to the operation of a stepper motor with each step. It is not possible to initiate a change whilst a stepper motor is mid-step. Therefore a motor control system which includes stepper motors includes inherent delays which are liable to cause accuracy to be limited to a certain extent.
Stepper motors are typically run in open loop control using microsteps to achieve the necessary step resolution. Angular rotor motion produced at each of the motor poles is guaranteed by the motor construction however the intervening positions produced by the microstepping cannot guarantee exact step size thus producing a positional error. This limits the ability of the tape drive to reduce the spacing between adjacent regions of tape from which ink can be removed, without risking overlapping images, which jeopardises print quality.
Known motor control systems provide accuracy which enables a user to print a series of images with a minimum spacing of 0.5 mm between adjacent portions of the tape from which ink has been removed by successive printing operations. The exact size of the spacing will be dependent upon many factors including the size of the image, the speed and acceleration of the substrate and the quality of the ribbon reel used in the printer.
A further example of a known tape drive includes a pressure roller in the tape path, which is driven by a motor. The roller directly controls the speed and position of the tape. The tape spools are driven through a mechanical clutch which maintains the tape tension between acceptable limits. Such tape drives are mechanically complex. The tape drive is typically uni-directional and this tends to cause tape wastage.
A still further example of a known tape drive is one in which two DC motors are used to drive the spools of tape (as described in FR 2783459, for example). Both of the motors operate in torque control mode and a roller which is positioned near to the printhead is used to determine the movement of the tape along the tape path. Such a tape drive includes rollers on the inked side of the tape which can require regular maintenance. Furthermore, desired printing speeds and tape accelerations are increasing leading to difficulties in operating such a drive.